30 Containment systems Cracks

Owing to hydrogen embrittlement, the mechanical properties of metallic and nonmetal-lic materials of containment systems may degrade and fail resulting in leaks. Hydrogen embrittlement depends on many factors such as environmental temperature and pressure, purity of metal, concentration and exposure time to hydrogen, stress state, physical and mechanical properties, microstructure, surface conditions, and the nature of the crack front of material [23]. 

In the case of a screw closure, it should be evident that a closure which has one turn or more of thread engagement will suffer less thread-to-thread stress than one of, say, turn (i.e. the same force (torque) is spread over less area with the latter, hence stress cracking could be more likely). This torque force is also conveyed to the sealing surface of the container and the contact surface of the closure, hence areas receive and convey a greater force (onto the cap). Studying how a stress occurs with the closure—container system is therefore a useful part of avoiding stress cracking problems. 

The total potential energy of the system containing a crack is therefore ... 

It is known [5], that at elastoplastic behavior a system crack-local deformation zone deviates from thermodynamical equilibrium and for its analysis a principles, correct for close to equilibrium systems, for example, Griffith theory, are inapplicable. Besides, prefailure zone structure is differed from elastically deformed material structure (Fig. 5.3) that complicates additionally process analysis. As it was noted above, for polymers this effect is displayed as the formation of local deformation zones near crack tip, containing microvoids and oriented material (crazes) or oriented material only (ZD) [20]. Therefore, for fracture analysis in such cases fracture fractal theory is applied, using fractal analysis and general principles of synergetics [28]. 

A very important part of a cracking unit consists of the lube and seal oil systems. In addition to the pumps and boiler uses, a cracking unit will have large systems for the air compressors and cyclone separators, which these units contain. Systems of up to 6000 gal (22,710 L) are not uncommon. 

The fracture mechanics (in a narrow sense) is usually associated with the mechanics of deformable bodies containing macroscopic cracks. These cracks are presented as mathematical cuts. The principal problem of fracture mechanics is to find conditions of stability of the system "cracked body - loading" with respect to the crack growth. 

Seismk Design of Waste Containment Systems, Fig. 2 Damage to landfills observed following the Northridge earthquake (a) cracking in soil cover on the... 

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